The present invention relates to an optical system capable of varying the spot diameter of a recording light beam during the recording operation, and particularly to an optical beam scanning apparatus suitable for use in a laser printer which performs multi-tone printing.
Conventional laser printers are designed to print multi-tone images in the form of a number of circular beam spots as shown in FIG. 1a. In the figure, beam spots shown by hatched circles move from left to right as shown by the arrows. A dark print portion has a high dot density and a bright print portion has a low dot density. In order to achieve finer multi-tone printing based on this method, it is necessary to use beam spots of smaller diameter and increase the number of beam scanning lines and the number of modulating operations. However, because of the mechanical beam scanning operation using a rotary multi-plane mirror or the like, the scanning speed cannot easily be increased. In case the number of scanning lines is increased, it adversely results in a slower printing speed. On the other hand, when the number of modulating operations is increased, it increases the computation time for the control, resulting ultimately in a slower printing speed.
There has been proposed a method of overcoming the above-mentioned problems, in which beam spots elongated in the second scanning direction are used for dark print portions (large solid areas) as shown in FIG. 1b. For example, a method is designed to vary the beam spot diameter by placing an electro-optical crystal, with electrodes being attached thereto, on the light path and applying an electric field to the crystal, as described in U.S. Pat. No. 4,768,043 and U.S. patent application Ser. No. 7/631257 (filed on Dec. 20, 1990 in connection with Japanese Patent Application Laid-Open No. 3-196023) now U.S. Pat. No. 5,196,697. Another method is designed to vary the beam spot diameter by placing a device of generating dynamic diffraction gratings of the surface acoustic wave (SAW) on the light path and varying the distance of gratings so that the diameter of beam spots formed of the 0th and .+-.1st order light is varied, as described in U.S. Pat. No. 5,053,619. Another method is designed to vary the beam spot diameter by using a semiconductor laser source, as described in U.S. patent application Ser. No. 7/747,189 (filed on Aug. 19, 1991) now U.S. Pat. No. 5,241,554.
The above-mentioned conventional methods are designed to operate at a constant optical power of laser, and therefore they are deficient in that the peak level of optical power varies when the beam spot diameter is varied. For example, a laser beam printer has its drum given optical sensitivity characteristics as shown in FIG. 2, and the peak power decreases as the beam spot diameter is increased. Accordingly, as shown in FIG. 3, printed dots 20a and 20b become smaller as the power decreases. This means that the elongated dot area becomes smaller than the circular dot area in FIG. 1b, that is opposed to the intention, and the print quality is degraded significantly.